1. Field of the Invention
The present invention relates to a scrambled transmission system including a scrambling apparatus for scrambling signals used in the transmission and storage of digitally coded signals to restrict who can reproduce the scrambled signals by providing the decoding procedure to authorized parties only and a descrambling apparatus for reproducing said scrambled signals and, more particularly, to a scrambled transmission system featuring high recoverability during signal reproduction free from errors occurring prior to signal reproduction.
2. Description of the Prior Art
An example of a conventional scrambled transmission system has a scrambling apparatus including a random number generator, an exclusive-OR circuit, and a multiplexer for multiplexing the key to the scrambled signal and transmitting the multiplexed signal; and a descrambling apparatus including a demultiplexer for separating the key, a random number generator, and an exclusive-OR circuit.
In this conventional scrambled transmission apparatus, the transmission signal is sequentially input to one input of the exclusive-OR circuit, and the output from the random number generator is input to the other input. The scrambling key is applied at each refresh cycle to the random number generator, which re-initializes using the scrambling key to output a random number sequence at a rate equal to the source signal rate. The source signal is therefore scrambled by the exclusive-OR operation using the random number generated by the random number generator. The scrambling key input to the random number generator is multiplexed by the multiplexer to the transmission signal, and transmitted.
At the receiver side, the scrambling key is extracted from the signal transmitted by the multiplexer, and the scrambled signal is input to the exclusive-OR circuit; the extracted scrambling key is input to the random number generator. The random number generator re-initializes using the scrambling key, generates the same random number sequence as that used on the transmission side, and inputs the random number sequence to the exclusive-OR circuit at the same rate as the scrambled signal. As a result, the same random number used on the transmission side is applied to the scrambled signal, thus restoring the source signal and producing the reproduction signal.
In these applications, the scrambling key is often encrypted using a higher-order encryption key, and this encryption key is decrypted on the receiving side to obtain the scrambling key. Because the random number generator is re-initialized using the scrambling key, synchronization can be restored when the random number generator is re-initialized using the next scrambling key in the event synchronization is lost between the transmission and receiving sides during the descrambling process. However, if a simple bit error occurs in the transmitted signal, only the error bit will be incorrectly reproduced, and all remaining bits will be correctly reproduced.
With the preceding apparatus, however, when the source signal is a signal containing variable length coding, and the reproducible audio and video are controlled by applying the signal scrambling process only to a specific code, e.g., the scramble effects code used in pay-per-view broadcasting, the bits and the number of bits that are scrambled will not be correctly detected by the receiver, or the scrambling position will be indeterminate, when a bit error occurs during transmission. As a result, synchronization of random number generator operation will be lost, and will not be recoverable until the random number generator is re-initialized using the next scrambling key.
Therefore, to achieve a bit error propagation rate in scrambled signal reproduction equivalent to that when the signal is not scrambled, it is necessary to synchronize the cycle for scrambling key refreshing to the cycle for decoding the variable length coding. In an MPEG-standard video signal using variable length coding, however, this cycle is less than 10 msec., which is unrealistic as a scrambling key refreshing cycle when it is considered that the scrambling key refreshing cycle in conventional scrambling systems is approximately 1 sec. At a realistic scrambling key refreshing cycle, however, reproduction recoverability drops significantly when bit errors occur. In addition, when only part of the data code is scrambled, such as when scramble effects control is used, a random number value is only required for the number of coded bits when the scrambled signal is obtained, but the complete random number sequence with a number of bits equivalent to the number of code bits must be available at the same time.